The invention relates to electronic amplifiers and more particularly to isolation amplifiers having substantially no DC conductive paths between the input terminals and any of the surrounding ground, output, for power supply circuits. More particularly still, the invention relates to such amplifiers including circuit features to improve the fidelity with which the input signal is relayed to the output. This invention particularly relates to improvements in such isolation amplifiers as may be commonly utilized in the medical field.
Amplifiers have in the past been provided with circuit arrangements for effecting conductive isolation between different components or elements connected thereto. Although such amplifiers have served useful functions in some fields, typically they have not been capable of meeting the severe requirements of a number of unique and important applications. One such important application is in the medical field where for a variety of purposes electronic equipment must be connected to human patients to measure electrical impulses and the like, e.g. for taking electrocardiograms. It has become increasingly apparent that the conventional electronic equipment can, when connected to a human, cause serious injury or even death in the event of minor equipment malfunctions, operator error, or some other inadvertent event. Further, there are many other applications for high performance isolation amplifiers as for instance in the fields of industrial process control where it becomes necessary to isolate the signal source from the system output.
One such isolation amplifier which generally meets the aforementioned requirements for isolation in the medical and/or process control fields is described in U.S. Pat. No. 3,946,324. That isolation amplifier comprises an input portion for receiving the DC input signal and includes, in the input portion, an AC energized modulator to produce an AC signal substantially corresponding to the DC input signal. The amplifier further includes an output portion receiving the AC signal from the input portion, which output portion also includes a phase sensitive demodulator to produce a generally corresponding, relatively amplified DC output signal. A transformer serves to non-conductively couple the AC signal from the input portion to the output portion. The AC signal from the modulator and the phase sensitive detection thereof by the demodulator are controlled by an AC energizing signal from an AC power portion of the amplifier. The AC power portion includes circuitry for converting a DC power source to the AC energizing signal and further includes a transformer for non-conductively coupling the AC energizing signal to the input portion of the amplifier in order to activate the modulator. Further still, the AC energizing signal is connected or coupled to the demodulator in an appropriate phase relationship with the AC signal extended to the modulator.
The isolation amplifier of the aforementioned patent, while generally providing the degree of isolation required in the medical and other fields may, under certain circumstances, permit a degree of degradation of the signal transferred from the input to the output which may be unsuitable to the user's needs. For example, in the field of electrocardiography even very slight distortions in the output signal may be misinterpretted as normal or abnormal as the case may be. Because of the importance in interpreting such ECG signals, it is particularly important that the output signal be a faithful reproduction of the input signal from the patient's heart.
In accordance with the present invention, it is believed that such signal degradation occurs because the AC energizing signal which is generally of the shape of a square wave is generated in a manner which permits a disturbing effect thereon by high frequency common mode noise signals present in the operating environment. Such noise signals may exist in the operating environment and be picked up by the leads connecting the patient to the amplifier.
Accordingly, it is a principal object of this invention to provide an isolation amplifier of the type described and possessing improved fidelity in the transfer of the input signal to the output.
It is another object of the present invention to control the generation of the AC energizing signal in a manner which minimizes the adverse impact of high frequency common mode noise on the transfer fidelity of the amplifier.
These and other objects will be in part obvious and in part pointed out in greater detail hereinafter.